Thinking in C++ learning(3)-----隐藏实现

1.访问控制：
public, private, protected

2.友元：
struct X{
private:
    int i;
friend void g(X*, int);      //Global friend
friend void Y::f(X*);         // Struct member friend
friend struct Z;                // Entire struct
};

void g(X* x, int i){
    x->i=i;
}

3.不完全的类型声明。时刻想着编译器怎么处理程序的。例如Y::f() 就只能用指针引用。
struct X;

struct Y{
    void f(X*);
};

struct X {
private i;
}